Pile leaching of gold and silver ores has been increasing in popularity since pile leaching techniques can be practiced with low capital and operating costs, and may be cost-effective for use with low grade ores. "Pile leaching" as used in the present specification and claims covers what are conventionally known as heap leaching processes, vat leaching processes, and like processes in which a pile of ore particles or the like have a cyanide leaching liquid applied thereto, with recovery of pregnant liquor from the bottom of the pile. The term "ore" as used in the present specification and claims covers tailings, uncrushed ore, crushed ore, agglomerated crushed ore, and the like. Pile leaching normally encompasses the percolation leaching of relatively coarse gold-siver ore piled on a surface which allows collection of the pregnant liquor obtained from the percolation leaching.
According to the invention it has been found that the leach rate can be increased, and/or the recovery can be increased in the same total leach time, by utilizing oxygen in the leaching process. That is oxygen containing gas, having a significantly higher precentage of oxygen therein than is obtained utilizing ambient air, is supplied to the pile. This may be practiced by introducing oxygen containing gas, such as "pure oxygen" (e.g. gas having about 99 percent oxygen) into bottom portions of the pile utilizing a plurality of pipes having gas passages therein. Additionally, or alternatively, the oxygen can be supplied to the pile by adding the oxygen to the cyanide leaching liquid that is applied to the pile to leach the gold and/or silver from the ore into the pregnant liquor. An ejector may be utilized to add the oxygen gas to the liquid prior to applying it to the pile, as by spraying it on top of the pile (particularly where heap leaching is utilized), flooding the top of the pile (particularly where vat leaching is practiced), or applying it as a foam (the oxygen gas and cyanide leaching liquid foaming) on top of the pile.
While the invention is applicable to a wide variety of "ores", as that term is used in the present specification and claims, it is particularly advantageous for use in leaching gold from those ores which typically consume oxygen and therefore tend to deplete the oxygen from leached solutions in conventional systems. Where agglomeration of the ore particles is desirable, that may be practiced utilizing any desirable conventional technique, such as shown in U.S. Pat. No. 4,256,705.
The gold and/or silver may be recovered from the pregnant liquor utilizing a number of conventional techniques. For instance the pregnant liquor may be withdrawn from adjcent the bottom of a heap and then treated by a carbon-adsorption system or a zinc precipitation system, or it may be withdrawn from the bottom of a rock filter of a vat leaching apparatus, and similarly treated by carbon-adsorption or zinc precipitation.
By practicing the present invention, it is possible to significantly increase the leach rate, or to increase recovery in the same total leach time, or a combination of both. Because of the increased leach rate that may be obtained by utilizing oxygen in the practice of the invention, the flow of solution to a pile could be increased to maintain the same gold concentration in solution, or, alternatively, the flow could be maintained at the same rate as in conventional pile leaching, resulting in a higher concentration of gold in solution (and thereby permitting a smaller recovery system).
It is the primary object of the present invention to enhance the effectiveness of pile leaching of gold and silver ores. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.